1. Field of the Invention
The present invention relates in general to solid-state imaging devices, and in more particular to charge-coupled device (CCD) image sensors. The invention also relates to highly integrated semiconductor imagers for use in small size photoelectric equipments such as video movie cameras, electronic still cameras or the like, the imagers providing an electrical signal indicative of an incident image introduced from a scene.
2. Description of the Related Art
With the increasing needs for high performance and "down-sizing" of photoelectric equipments such as home-use video cameras, personal electronic still cameras or the like, development of a solid-state image sensing device with further enhanced integration density has been demanded strongly. As the number of cells increases on a semiconductor substrate of limited size, the cell size decreases, the magnitude of the charge storage (charge handling capability) in each cell of necessity decreases. These factors reduce the sensitivity, which may lead to a decrease in the quality of sensed image.
The presently available charge-coupled device (CCD) image sensors include an array of rows and columns of picture elements or cells each having a diode for storing therein a packet of photoelectrically produced signal-charge carriers indicative of an incident image introduced from a scene. A plurality of charge transfer sections called the "vertical CCD shift registers" extend along the columns of cells on the substrate in such a manner that each vertical CCD shift register is associated with a corresponding one of the columns of cells. The vertical CCD shift registers are coupled at their outputs to another charge transfer section that extends perpendicularly to the vertical CCD shift registers, which is known as the "horizontal CCD shift register" among those skilled in the art to which the invention pertains.
In other words the prior art CCD imagers should require the presence of one vertical charge transfer section with respect to the photosensitive section of each of a number of cells. This necessitates the alternate positioning of the vertical charge transfer sections and the columns of cells on the substrate of a limited surface area. It should not be permissible that such transfer sections are simply decreased in width in order to provide the charge transfer performance of necessity. The prior art imagers are arranged so as to cause the occupation ratio of the vertical transfer sections on the substrate surface to be greater than a predetermined allowable value. This is due to the fact that, if the width of vertical transfer sections is decreased to increase the effective substrate surface area used for the layout of cell array thereon in order to achieve a further integration density, the resultant signal-charge transferring performance of the vertical transfer sections toward the horizontal charge transfer section is decreased, which leads to the impossibility of transferring sufficient sensed signal charge packet. This is a serious bar to the achievement of further integration density of CCD imagers. There is a trade-off between the accomplishment of further integration density and that of higher quality of a reproduced image; the conflicting objectives have forced experts in the art to suffer from walking the long and winding road of comprise.
To enhance the integration, a photoconversion layer overlaid image sensor has been developed, which may be called the "photoconductive-layered solid-state imaging device" (PSID). With the imager of this type, an amorphous semiconductor photoconductive film is stacked over a substrate to cover an array of rows and columns of cells as a whole. The film may internally produce a packet of signal charge carriers corresponding to an optical image introduced to the film, which carriers are then supplied by lead electrodes to respective cells each having a PN-junction diode for charge storage.
According to the photoconversion layer overlaid CCD imager, the aperture ratio of photodiodes is almost 100 percent, causing the cell size of necessity to decrease accordingly. The effective substrate surface area utilized for the layout of cells can thus be increased; this permits an increased number of cells to be mounted on the same substrate. The integration density increases. However, by taking account of the fact that the requirement of higher integration will continue endlessly in the art of CCD imagers, the advantages of the CCD imager structure will reach a limit in the near future.